Q: How do I tell the difference between a break line popping and a master cylinder going bad while my foot was on the brake?

You may have an internal piston seal leaking inside the master cylinder or a leak somewhere in the brake system. You would know the difference by looking in the master cylinder to see if the fluid is real low or empty. If it is then the master cylinder most likely is not the problem but a leak in the brake system somewhere is. I recommend having the brakes checked to see if you see any evidence of a leak under the vehicle if the fluid is gone out of the master cylinder. If the front brake calipers leak you will see fluid on back side of a wheel. If it is a brake line leaking then it may be due to rusted through lines and may need multiple line repairs to fix the rusted lines. Have a brake safety inspection to check the entire system integrity.